Pulse modulation schemes encode data as a series of digital pulses according to an input analog signal. One advantage of pulse modulation is that power loss in the switching devices is very low. Many switched amplifiers use a full-bridge configuration, which can provide positive or negative current to the load without a negative power supply or a DC (direct current)blocking capacitor. FIG. 1 is a graph illustrating various conventional pulse modulation schemes for full-bridge output stage. Data 102 may be encoded into a waveform 104 according to an out-of-phase modulation scheme. The waveforms 104A and 104B may be output to two nodes, and used to drive a load, such as a speaker or an antenna. Data 102 may alternately be encoded into a waveform 106 according to an in-phase modulation scheme. Data 102 may further be encoded into a waveform 108 according to a single-sided modulation scheme.
Modulated data may, in some systems, be applied to a load to convey the data. For example, the modulated data may be output to a speaker when the encoded data is audio data to deliver audio sounds to a user of a device. In certain applications, it is may be desirable to monitor the current that is flowing through the load with high precision. FIG. 2 is a circuit diagram illustrating a conventional amplifier arrangement for a speaker with current monitoring. A circuit 200 includes a speaker 202 coupled to a first driver 204 and a second driver 206. A resistor 208 may be coupled between the speaker 202 and the first driver 204 to allow current monitoring through the speaker 202. A current monitor 210 is coupled to the resistor 208 to measure, for example, a voltage across the resistor. The current monitor 210 may include diodes 212 to provide protection, such as electrostatic discharge (ESD) protection. The diodes 212 may also include parasitic diodes associated with any switch inside the current monitor 210.
The drivers 204 and 206 may receive input waveforms having modulated data and amplify the input waveforms to produce output at the speaker 202. For example, the drivers 204 and 206 may switch transistors 204A or 206A to an on state to connect a first output node, Outp, or a second output node, Outm, respectively, to a positive voltage rail, VDD. The drivers 204 and 206 may also switch on transistors 204B or 206B to connect the first output node, Outp, or the second output node, Outm, respectively, to a negative rail, such as ground. The difference between the first output node, Outp, and the second output node, Outm, generates a current through the speaker 202, which conveys the waveform to the user as an audible sound.
When the input waveforms to the drivers 204 and 206 are encoded according to conventional encoding schemes, such as those illustrated in FIG. 1, the output signals outp and outm may exceed a positive supply voltage, VDD, or fall below a negative supply voltage, gnd, which reduces the accuracy of the current monitor 210. First, during a high-to-low or low-to-high transition of outp or outm, overshoot and undershoot may occur, which may cause outp or outm to exceed VDD or fall below gnd. Second, during a steady state of outp and outm, outp and outm may exceed VDD or gnd. For example, when outp is connected to VDD for a period of time while load current at the time is flowing into the outp node, then outp may exceed VDD for a voltage of Iload*ZPMOS, where Iload is the current through the speaker 202 and ZPMOS is the impedance of the PMOS components of the drivers 204 and 206. Likewise, the same occurs when outp is connected to gnd but current flows out of outp. Depending on the load current amplitude and switch size, the overshoot or undershoot during the steady-state period may exceed one volt. If the output node, outp, which is coupled the current monitor 210, exceeds VDD or below gnd, precision in the current monitor 210 may be lost. Forward-biasing the diode 212 may create current that will be sensed by the sense resistor 208 but does not actually flow to speaker 202, thus reducing the precision of the current monitor 210. If there are switched capacitor circuits used as an interface for the current monitor 210, charge spill may happen as the result of outp exceeding VDD or below gnd, which may affect the precision of the current monitoring circuit.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved integrated circuits, particularly for consumer-level devices. Embodiments described here address certain shortcomings but not necessarily each and every one described here or known in the art.